Acetal polymers have been in commercial use for many years. These polymers have oxymethylene repeat units as a major part of their backbone. They are widely used in molding compositions, which have been utilized in many applications, such as automobile bumper extensions and instrument panels; plumbing supplies, such as valves, shower assemblies, flush tank components, faucets and pipe fittings; tool components; and household and personal products.
These crystalline acetal polymers have excellent physical properties. However, for certain applications, improved impact resistance would be highly desirable. Typically, impact strength of a crystalline polymer is improved by blending the crystalline polymer with an elastomer to form shock-absorbing rubbery domains in the crystalline polymer. This approach to impact resistance is most successful when there is a strong interaction between the surface of the rubbery domains and the crystalline polymer.
A limited number of acetal copolymers have recently been synthesized wherein the compositions yield a noncrystalline acetal. The chemical structures are similar enough to the crystalline acetals to interact well with them. For example, commonly assigned U.S. Pat. No. 4,788,258 discloses acetal copolymers derived from trioxane and 1,3-dioxolane, with the 1,3-dioxolane content being between about 65 and 75 mol percent of the polymer based on the total monomer composition. These polymers have a glass transition temperature that is less than about -60.degree. C. Blends of these non-crystalline copolymers with crystalline acetal polymers show improved impact resistance over that of the unblended crystalline acetals. Copending and commonly assigned U.S. Ser. No.406,641 discloses acetal copolymers made from 1,3-dioxolane and 1,3-dioxepane. These copolymers are non-crystalline and have glass transition temperatures which are as low as -120.degree. C. and below. Blends of these copolymers with crystalline acetal polymers also exhibit improved impact resistance.
The elastomeric acetal polymers described above are thermoplastic materials which readily deform under stress. Very few elastomeric acetal polymers have been reported which are crosslinked or can be crosslinked. One example of a crosslinkable acetal polymer has been disclosed in commonly assigned U.S. Pat. No. 4,758,608. This polymer is synthesized from trioxane, 1,3-dioxolane, and a formal of a monoethylenically unsaturated aliphatic diol. This polymer can be cured with a multifunctional crosslinking monomer in the presence of ultraviolet light to yield an insoluble, rubbery, non-tacky polymer which is useful for blending with crystalline acetal polymers.
A crosslinked elastomeric acetal polymer composition has also been disclosed in commonly assigned U.S. Pat. No. 4,898,925. The polymer is made from trioxane, 1,3-dioxolane, and about 0.005% to about 0.15% of a bifunctional monomer, such as 1,4-butanediol diglycidyl ether or butadiene diepoxide. This polymer exhibits improved elastomeric properties. When it is blended with crystalline acetal polymers, the blend has improved properties compared with the properties of the unblended acetal polymer.
Additional elastomeric acetal compositions are needed, especially ones that are crosslinked so that they are not thermoplastic. These are particularly useful for the purpose of expanding the range of impact-modified blends of crystalline acetal polymers. A novel, crosslinked elastomeric acetal is disclosed herein. The fact that this material is non-crystalline is particularly surprising and unexpected, since the analogous composition which does not incorporate a crosslinking monomer is crystalline.